1 Field of the Invention
The invention relates to a piezoelectric sensor, preferably a pressure sensor, with at least two piezoelectric measuring elements placed in a housing, which are clamped between a membrane on the pressured side of the housing and a pick-up electrode which is electrically insulated against the housing.
2 The Prior Art
Piezoelectric measuring elements serve for measuring forces, pressures, acceleration, strain and moments, for instance, and are used in sensors which are optimized for the application desired. If the transversal piezoeffect is employed measuring elements consisting of thin plates or rods are used, the force or pressure to be measured being applied via the top faces or narrow sides. Since the sensitivity of the piezoelectric measuring element is proportional to the ratio between the area of the face on which charge is accumulated (side face) and the pressured area (narrow side), the crystal elements are relatively thin, and thus a number of such crystal lamellas are used parallel to each other in order to sustain the required pressure and to enhance sensitivity. This makes handling, in particular positioning and aligning, of the individual measuring elements rather complicated. Centering means are therefore often provided in the sensors, which are designed to hold the crystal elements in position. It is for instance known to position lamella-shaped measuring elements relative to each other by means of spacer sheets, these spacer sheets being formed in such a way that they act as springs on the crystal elements, pressing them against the inside wall of the housing, which is covered with electrically insulating plastic material. In the instance of certain pressure sensors which are inserted in indicator bores, the sensor housing has an interior diameter of about 2 mm only, leading to very small structures and thus to increased handling difficulties in manufacturing.
A further disadvantage inherent in the use of plastic positioning means is due to their increasing conductivity at high temperatures, which may cause the sensor to fail. The plastic material undergoes irreversible changes at very high temperatures (400° C.), leading to a substantial increase in conductivity and thus to electrical short circuits.
In this context a multilayer piezoelectric measuring element has become known from WO 2005/026678 A, which may be used in a pressure or force sensor. The sensor described in this document has at least two piezoelectric crystal elements placed directly side by side, and employ the transversal piezoelectric effect. The crystal elements are lamella-shaped and have a thickness of less than 0.5 mm. The individual crystal elements are serially arranged with alternating polarity and are rigidly bound together via their electrode layers by means of bonding, soldering or thermo-compression. The advantage of such a stack of bonded crystal elements lies on the one hand in its high sensitivity and stability under load. On the other hands introducing the stack into a sensor is at least partly simplified, since the whole stack, and not each single crystal element, is positioned accordingly and electrically insulated from the interior wall of the sensor housing.
From EP 1 283 552 A2 there is known a piezoelectric crystal with transversal effect which can be built into a sensor without centering and without the use of further assembly means. To this end, the piezoelectric crystal is furnished with a one-part base with individual round segments, which can be inserted into a bore of a holding part of the sensor. Furthermore, a variant is described in EP 1 283 552 A2, in which two mirror-symmetrical crystals are arranged side by side, resulting in doubled sensitivity of the sensor. Both the simple crystal with its base and the double crystal with a divided base made up of two mirror halves, are relatively complicated to manufacture, and the remaining handling effort for assembling the sensor is still considerable.
From GB 2 022 261 A there is known a piezoelectric pressure sensor with a two-part housing, which has a pestle-shaped pick-up electrode electrically insulated against the upper part of the housing, and where the single piezoelectric measuring element is glued to the pick-up electrode with its back and to a pressure plate with its front. When the pressure sensor is assembled and ready for use, the glue will inhibit movement between the measuring element and the contact face of the electrode; the possibility of such movement is required for stable measuring properties and is necessary to avoid destruction of the measuring element at high temperatures due to differing thermal expansion coefficients of the measuring element and the pick-up electrode. Glueing, as described in document GB 2 022 261 A, has further disadvantages, since at higher temperatures the insulation properties will deteriorate due to evaporating residual glue.